class ActiveStorage::DirectUploadsController

the blob that was created up front.
When the client-side upload is completed, the signed_blob_id can be submitted as part of the form to reference
Creates a new blob on the server side in anticipation of a direct-to-service upload from the client side.

def blob_args 

def blob_args
  params.require(:blob).permit(:filename, :byte_size, :checksum, :content_type, metadata: {}).to_h.symbolize_keys
end

def create 

def create
  blob = ActiveStorage::Blob.create_before_direct_upload!(**blob_args)
  render json: direct_upload_json(blob)
end

def direct_upload_json(blob) 

def direct_upload_json(blob)
  blob.as_json(root: false, methods: :signed_id).merge(direct_upload: {
    url: blob.service_url_for_direct_upload,
    headers: blob.service_headers_for_direct_upload
  })
end